How should we annotate cooperative inheritance?

[NeilGirdhar]

Consider annotating this example of cooperative multiple inheritance:

from typing import Any


class Hidden:

    def __init__(self, z: float, **kwargs: Any):
        super().__init__(**kwargs)
        self.z = z


class Base:

    def __init__(self, x: int, **kwargs: Any):
        super().__init__(**kwargs)
        self.x = x


class Inherited(Base):

    def __init__(self, y: int, **kwargs: Any):
        super().__init__(**kwargs)
        self.y = y


class Final(Inherited, Hidden):
    pass

MyPy gives errors because of the super-calls, which is addressed by #4001.

It would be nice for Inherited to ensure in its super-call that x has the right type, which is the topic of #4441 although it would also be nice for Final's constructor to only accept x, y, and z. I can't find any issue for that.

This example is done with __init__, but the problem can occur in any such “augmenting” method pattern. It is not restricted to __init__. This augmenting pattern always needs a top level method in some class. In __init__'s case, it's in object.

One idea is to have a special annotation on **kwargs, for example, typing.Collector? MyPy could then figure out the necessary arguments and their types in all derived classes. An inherited class would only be allowed to explicitly forward variables accepted by parents, but they should be able to forward anything. MyPy would make a best effort to ensure that all required arguments get in somehow.

I recognize that not everyone is a fan of cooperative MI, but I personally think it's quite beautiful. I would love to see MyPy handle it elegantly.

[jnoortheen]

This would help extending functions with large number of arguments.

[NeilGirdhar]

Can this issue be moved to python/typing?